Radio Amateur’s Sub-9 kHz VLF Signal Detected Across the Atlantic
How low can you go? A signal on 8.971 kHz has spanned the Atlantic, from North Carolina to the UK. Running on the order of 150 µW effective radiated power, very low frequency (VLF) experimenter Dex McIntyre, W4DEX, transmitted signals on June 2 and 3 that were detected by Paul Nicholson, an SWL in the UK. Earlier this year, Nicholson detected McIntyre's WH2XBA/4 Experimental Service VLF signal on 29.501 kHz. McIntyre needed no FCC license to transmit on 8.971 kHz, since the Commission has not designated any allocations below 9 kHz — dubbed “the Dreamers’ Band.”
“I’ll probably make more transmissions on 8.9 kHz when there is no chance of thunderstorms,” McIntyre told ARRL this week. “Then, maybe sliding down in frequency to see how low I can go for Paul to detect the signal.”
Right now, McIntyre said his priority is to complete a transverter for the other end of the spectrum — 5.7 GHz. He’s also working on a second 24 GHz system as a loaner, “so I’ll have someone to work,” he noted. “Sure is lonely here above UHF.”
Nicholson used sophisticated digital signal processing (DSP) software to detect McIntyre’s transmission. The distance was approximately 6194 km (approximately 3840 mi). Nicholson was able to detect the VLF signal during both daylight and nighttime paths.
On June 1 and into June 2, McIntyre transmitted a steady, GPS-locked carrier at 8.971.000 kHz. Between 0000 and 0600 UTC, Nicholson measured a carrier at that frequency in Todmorden, UK. He combined the electrical and magnetic field receiver outputs to produce a unidirectional antenna response.
“This brought the signal up to a significant level,” Nicholson reported. “The signal bearing was roughly west. W4DEX is bearing 285° from here. The S/N is max on a bearing of 315°, which puts the prevailing southwesterly background nearer to the side of the antenna response. This gave the signal a respectable 12.5 dB S/N in a 46 µHz bandwidth.” Nicholson said McIntyre’s carrier also was visible during a daylight path in a 23 µHz bandwidth.
Just to confirm that Nicholson was actually detecting McIntyre’s signal, the two experimenters conducted a blind confirmation test. McIntyre changed his transmit frequency slightly, and Nicholson was also able to detect that signal at 8.971.100 kHz at the same strength. An e-mail response from McIntyre confirmed 8.971.1 kHz as the new frequency.
Is a two-way sub-9 kHz contact in the offing? “A two-way, transatlantic contact would be a hundred times more amazing than just a simple signal detection,” McIntyre said. “I seriously doubt I will ever have that receive capability. But not long ago I seriously doubted I would be the first to receive a transatlantic or New Zealand 137 kHz transmission. Going down in frequency has been as much fun as going higher. It’s all RF.”
McIntyre expressed his appreciation to Nicholson, “who was convinced this could be done with so little radiated power and for his continuous prodding to give it a try.” He also credited Nicholson’s “amazing weak-signal software.
McIntyre’s transmitter consisted of a Hewlett Packard HP 3586B selective level meter with tracking generator. The low-level generator output is amplified by a Wandel & Golterman A-160 level regulator, which feeds a Hafler P3000 stereo audio amplifier, which has been bridge connected for mono output. In this configuration, the P3000 is capable of putting out 400 W of audio into an 8 W load. McIntyre said the same generator and amplifier have been used on 137, 74, and 29 kHz experiments.
His antenna is essentially the same one he uses for 160 meters and for other LF experiments. For this experiment, however, it was equipped with a gigantic base-loading coil, which contains nearly a mile of wire. “The vertical wire is spaced 1.5 meters from the tower, hanging from an insulator 29 meters above ground,” McIntyre explained. “Top hat consists of about 170 meters of #18 Copperweld. Most of the top hat wires run about 7 to 20 meters over the top of a combination of oak and pine trees. Total antenna capacitance is close to 1200 pF.”
McIntyre also thanked Jay Rusgrove, W1VD, and John Andrews, W1TAG, for their technical advice, Markus Vester, DF6NM, for technical advice “and additional prodding,” Mal Hamilton, G3KEV, for coming up with the term “Dreamers’ Band,” and Stefan Schaefer, DK7FC, for “proving dreams can come true.”
He also expressed gratitude to Warren Ziegler, K2ORS, for the opportunity to experiment on 29 and 74 kHz, “and all the other VLF experimenters who posted their results on the RSGB LF Group and the Yahoo Sub 9kHz Amateur Radio group.